diff --git a/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs b/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
index c43bd7b2c494e..b4fd9cc307492 100644
--- a/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
@@ -12,7 +12,7 @@ use rustc_type_ir::search_graph::{CandidateHeadUsages, LowerAvailableDepth, Path
 use rustc_type_ir::solve::{
     AccessedOpaques, ExternalRegionConstraints, FetchEligibleAssocItemResponse, MaybeInfo,
     NoSolutionOrRerunNonErased, OpaqueTypesJank, QueryResultOrRerunNonErased, RerunCondition,
-    RerunNonErased, RerunReason, RerunResultExt, SmallCopyList,
+    RerunNonErased, RerunReason, RerunResultExt, SmallCopySet,
 };
 use rustc_type_ir::{
     self as ty, CanonicalVarValues, ClauseKind, InferCtxtLike, Interner, MayBeErased,
@@ -1732,7 +1732,7 @@ fn should_rerun_after_erased_canonicalization<I: Interner>(
     parent_opaque_types: &[(OpaqueTypeKey<I>, I::Ty)],
 ) -> RerunDecision {
     let parent_opaque_def_ids = parent_opaque_types.iter().map(|(key, _)| key.def_id.into());
-    let opaque_in_storage = |opaques: I::LocalDefIds, def_ids: SmallCopyList<_>| {
+    let opaque_in_storage = |opaques: I::LocalDefIds, def_ids: SmallCopySet<_>| {
         if def_ids.as_ref().is_empty() {
             RerunDecision::No
         } else if opaques
diff --git a/compiler/rustc_type_ir/src/solve/mod.rs b/compiler/rustc_type_ir/src/solve/mod.rs
index a916dbd079efa..303476fa11694 100644
--- a/compiler/rustc_type_ir/src/solve/mod.rs
+++ b/compiler/rustc_type_ir/src/solve/mod.rs
@@ -79,17 +79,21 @@ impl From<RerunNonErased> for NoSolutionOrRerunNonErased {
     }
 }
 
+/// A small set of up to 3 `Copy` elements, used as an optimization in [`RerunCondition`].
+/// The entire set can be `Copy`ed because of this requirement.
+///
+/// Set properties maintained using [`union`](SmallCopySet::union), which deduplicates values.
 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
 #[derive(TypeVisitable_Generic, TypeFoldable_Generic, GenericTypeVisitable)]
 #[cfg_attr(feature = "nightly", derive(StableHash_NoContext))]
-pub enum SmallCopyList<T: Copy + Debug + Hash + Eq> {
+pub enum SmallCopySet<T: Copy + Debug + Hash + Eq> {
     Empty,
     One([T; 1]),
     Two([T; 2]),
     Three([T; 3]),
 }
 
-impl<T: Copy + Debug + Hash + Eq> SmallCopyList<T> {
+impl<T: Copy + Debug + Hash + Eq> SmallCopySet<T> {
     fn empty() -> Self {
         Self::Empty
     }
@@ -127,7 +131,7 @@ impl<T: Copy + Debug + Hash + Eq> SmallCopyList<T> {
     }
 }
 
-impl<T: Copy + Debug + Hash + Eq> AsRef<[T]> for SmallCopyList<T> {
+impl<T: Copy + Debug + Hash + Eq> AsRef<[T]> for SmallCopySet<T> {
     fn as_ref(&self) -> &[T] {
         match self {
             Self::Empty => &[],
@@ -165,12 +169,12 @@ pub enum RerunCondition<I: Interner> {
     /// Note that this only reruns according to the condition *if* we are in [`TypingMode::Typeck`].
     AnyOpaqueHasInferAsHidden,
     /// Note: unconditionally reruns in postanalysis
-    OpaqueInStorage(SmallCopyList<I::LocalDefId>),
+    OpaqueInStorage(SmallCopySet<I::LocalDefId>),
 
     /// Merges [`Self::AnyOpaqueHasInferAsHidden`] and [`Self::OpaqueInStorage`].
     /// Note that just like the unmerged [`Self::OpaqueInStorage`], that part of the
     /// condition only matters in [`TypingMode::Typeck`]
-    OpaqueInStorageOrAnyOpaqueHasInferAsHidden(SmallCopyList<I::LocalDefId>),
+    OpaqueInStorageOrAnyOpaqueHasInferAsHidden(SmallCopySet<I::LocalDefId>),
 
     Always,
 }
@@ -327,7 +331,7 @@ impl<I: Interner> AccessedOpaques<I> {
         debug!("set rerun if post analysis");
         self.update(AccessedOpaques {
             reason: Some(reason),
-            rerun: RerunCondition::OpaqueInStorage(SmallCopyList::empty()),
+            rerun: RerunCondition::OpaqueInStorage(SmallCopySet::empty()),
         })
     }
 
@@ -339,7 +343,7 @@ impl<I: Interner> AccessedOpaques<I> {
         debug!("set rerun if opaque type {defid:?} in storage");
         self.update(AccessedOpaques {
             reason: Some(reason),
-            rerun: RerunCondition::OpaqueInStorage(SmallCopyList::new(defid.into())),
+            rerun: RerunCondition::OpaqueInStorage(SmallCopySet::new(defid.into())),
         })
     }